In a digital content distribution service that is provided via a network, the service supplier manages the clients by, for example, requesting the user to present his/her age to judge whether or not the content to be provided is appropriate for his/her age, or requesting the user to present the address of his/her residence.
On the other hand, the user, who is requested to present his/her age or address, has a desire to present only minimum personal information. The below-identified Non-Patent Document 1 discloses a technology for satisfying the desire.
According to the technology, an attribute certificate issuance server that manages the personal information such as the name, address, and age of users, as well as public keys assigned to the users, issues an attribute certificate containing as many information as requested by the user. For example, when the user wants to certify only his/her age, the user requests the server to issue an age certificate. Upon receiving the request to issue the age certificate, the server issues attribute certificate data that is generated by applying a digital signature to combined data of a public key and age of the user. The certificate assures that the user is of the age contained in the certificate when the user has a private key corresponding to the public key contained in the certificate. In this example case, the attribute certificate discloses the age of the user, but does not disclose the other information such as the name and address. That is to say, the user can disclose only necessary personal information.    Patent Document 1: Japanese Patent Application Publication No. 2004-289247    Non-Patent Document 1: ITU-T, “Information Technology—Open systems interconnection—The directory: Public-key and attribute certificate frameworks”, Recommendation X. 509, March 2000